1. Field of the Invention
This invention relates generally to scanning probe microscopy and more specifically to scanning evanescent near field microwave and electromagnetic spectroscopy.
2. Description of the Related Art
Scanning probe type microscopes have typically been used to create visual images of a sample mail. The image obtained may reflect any of a number of distinct electrical or magnetic properties of the sample material, depending on the parameter measured by the probe tip. For example, the tip may image electron tunneling, atomic force, absorption and refaction of propagating or evanescent electromagnetic waves, or other parameters. The tip may be in contact with the sample or it may be a short distance above the sample. A thorough discussion of scanning probe microscopes is presented by R. Wiesendanger, xe2x80x9cScanning Probe Microscopy and Spectroscopy: Methods and Applicationsxe2x80x9d Cambridge University Press, 1994. Efforts in improving Scanning Probe Microscopes (SPMs) have focused almost entirely on increasing their resolution and sensitivity. While it is generally recognized that obtaining quantitative data to associate with the image detail would be highly desirable, two major technological barriers have prevented such instruments from being developed.
First, microscopy signals, as obtained from SPMs often are a combined function of topography and physical properties of the material. Separating them requires measuring at least two independent signals. For example, in scanning tunneling microscopy, the tunneling current is a function of both the tip to sample distance and the density of states. A recently developed scanning near-field optical microscope can measure optical signals such as luminescent spectra or optical index of refraction in addition to shear force, which can be used to determine the distance between tip and sample.
Second, to obtain quantitative information regarding the physical sample being imaged, complicated electromagnetic field equations in the region of the tip and sample must be solved. A review of this work is discussed by C. Girard and A. Dereux in Rep. Prog. Phys., vol. 657, 1996. Although numerical methods based on finite element analysis have been used to solve the field distribution around a near-field optical microscope tip, the complicated computational process involved, such as solving the Maxwell equations under real boundary conditions on a scale of a wavelength or less, is not practical in routine applications. The problem has been complicated for the work done in the past, because the microscopes were required to operate below a cut-off frequency and so suffered severely form waveguide decay, having a typical attenuation of 10xe2x88x923 to 10xe2x88x926 (R. F. Soohoo, J. Appl. Phys. 33:1276, 1962; E. A. Ash and G. Nichols, Nature, 237:510, 1972). In aperture or tapered waveguide probes, a linear improvement in resolution causes an exponential reduction in sensitivity. M. Fee, S. Chu, and T. W. Hansch, improved sensitivity and resolution to the micron level (Fee, M. et al., Optics Commun., 63:219, 1988) by using a transmission line probe with a reduced cross-section. However, further improvement in resolution was still accompanied by significant transmission line decay. The unshielded far-field wave propagation components around the tip of the transmission line probe significantly limited the resolution of the microscope, and particularly interfered with its use for quantitative analysis.
It would be highly desirable to have a scanning probe microscope capable of making images of features having submicron resolution and additionally capable of making quantitative measurements of the physical properties of the imaged features.
The invention comprises a near field scanning evanescent-wave microscope wherein a probe tip primarily emits an evanescent wave and wherein interfering propagating wave emissions are m ms Propagating waves have low resolution while evanescent waves have high resolution. This feature is crucial for quantitative measurements, where only the near-field evanescent wave is modeled. A high resolution image is generated by scanning a sample with a novel evanescent wave probe on the inventive microscope. Furthermore, the inventive microscope provides complex electrical impedance values that are calculated from measured data and which are associated with the resolved image features. The complex impedance, including dielectric constant, loss tangent and conductivity can be measured for materials having properties that range from insulators to superconductors.
The inventive microscope is capable of quantitative measurements of dielectric properties and surface resistance with submicron resolution. By monitoring the resonance frequency (fr) and quality factor (Q) of a resonant coaxial cavity coupled to the tip, the electrical properties of the sample are measured. One embodiment of the SEMM comprises a xcex/4 coaxial resonator operating at frequency (fr) of roughly 1 GHz coupled to a sharp tip protruding from a narrow hole. When the probe tip is brought near a sample, fr and Q shift. The inventive microscope is capable of converting the measured fr and Q shifts to electrical parameters of the sample. Since the extremely small tip radius determines the extent of the field distribution, this microscope is capable of submicron resolution. For dielectric samples, the interaction between the probe tip and the sample is dependent on the dielectric constant and tangent loss of the nearby sample. For a metallic sample, the interaction depends on the surface resistance of the sample.
The probe itself, comprising either a resonator or a conventional coaxial body, is a key inventive feature of the microscope. An important novel feature of the probe tip is a conducting endwall having an aperture, through which the center conducting element of the coaxial cable or resonator extends without shorting to the endwall. Another key feature of the inventive microscope is the computing element programmed to convert measured changes in resonant frequency (or reflected electromagnetic wave) and measured changes in the quality factor to quantitative electrical parameters of the sample. An additional important feature of the inventive microscope is a means to maintain a constant separation distance between the tip and the sample while measurement scans of the sample are performed.